1. Field of the Invention
The present invention relates to an image pickup apparatus such as a video camera or an electronic still camera, and more particularly to the white balance control function thereof.
2. Related Background Art
A video camera or the like usually incorporates a white balance correcting device, in order to obtain an appropriate balance of different colors at the image taking operation. FIG. 1 is a block diagram of a prior art video camera including an automatic white balance correcting device, disclosed in a U.S. patent application Ser. No. 070,493 of the present applicant. There are shown an image pickup device 1; a circuit 2 for generating luminance and chromaticity signals; gain control circuits 3, 4 respectively inserted in red (R) and blue (B) signal lines; a color difference signal generating circuit 5; an encoder 6; gate circuits 7, 8; an (R-B) signal detection circuit 9; an averaging circuit 10; a differential amplifier 11; a limiting circuit 12; a tracking correction circuit 13; and a lens 15, wherein the gain control circuits 3, 4 and the circuits 7 to 13 constitute an automatic white balance correction device 14.
The above-explained image pickup apparatus functions in the following manner. Light entering the image pickup device 1 is photoelectrically converted therein, and the obtained signal is supplied to the luminance signal-chromaticity signal generating circuit 2, which generates a high frequency component YH of the luminance signal, a low frequency component YL of the luminance signal, a red signal (R) and a blue (B) signal. Among these signals, the R and B signals are respectively amplified in the gain control circuits 3, 4, according to the characteristics controlled by control signals from the tracking correction circuit 13, to provide color signals R', B' which are supplied, together with a luminance signal YL, to the color difference signal generation circuit 5 for generating color difference signals (R-YL) and (B-YL). Said color difference signals are supplied, together with a luminance signal YH, to the encoder 6 for conversion into a standard television signal. Said color difference signals (R-YL) and (B-YL) are also supplied to the automatic white balance correction device 14. In said device, the color difference signals are respectively supplied to the gate circuits 7, 8 for eliminating an unnecessary signal in the blanking period, an abnormal color difference signal resulting from a signal saturation for a high luminance object etc.
The signals released from said gate circuits 7, 8 are supplied to the (R-B) signal detection circuit 9, which generates an (R-B) signal by calculating the difference of the (R-YL) and (B-YL) signals from the gate circuits 7, 8. The averaging circuit 10 averages the (R-B) signal from the (R-B) signal detection circuit 9, thereby obtaining a DC signal. The differential amplifier 11 compares the output signal level from the averaging circuit 10 with a reference voltage V.sub.ref1, and generates a signal corresponding to the difference, for supply to the limiting circuit 12, which limits the level of the output signal from the amplifier 11 within a range between V.sub.2r and V.sub.3r, respectively corresponding to the upper and lower limits of color temperature, in order that the white balance is controlled within a practical color temperature range (for example 2000.degree. to 10000.degree. K.). Thus the output from the limiting circuit 12 is limited within the range from V.sub.2r and V.sub.3r.
The output signal of the limiting circuit 12 is supplied to the tracking correction circuit 13, in response, provides the gain control circuits 3, 4 with control signals R.sub.cont, B.sub.cont for controlling the gains of the gain controlling circuits 3, 4 so as to correct the white balance, namely for reducing the (R-B) signal component of the object to zero.
In the following there will be explained an example of the relation between the signals R.sub.cont, B.sub.cont and the color temperature, with reference to FIGS. 2 and 3.
In a vector chart shown in FIG. 3, points X1, X2 and X3 respectively correspond to white color at 6000.degree. K., 2000.degree. K. and 10000.degree. K. If the control signals R.sub.cont, B.sub.cont have voltages V.sub.1r, V.sub.1b corresponding to X1 as shown in FIG. 2, said control signal will have voltages V.sub.2r, V.sub.2b for correcting X2 toward the center of the vector chart, and voltages V.sub.3r, V.sub.3b for correcting X3 toward said center.
Since the control signals R.sub.cont, B.sub.cont are limited to V.sub.3r, V.sub.3b, an image corresponding to a point X4 in FIG. 3 cannot be corrected to the center X1 of the vector chart even under the function of the white balance correcting device.
Since the automatic white balance correction device 14 has a negative feedback loop explained above, color difference signals with a white balance can be supplied to the encoder 6 within a practical color temperature range.
However the above-explained conventional white balance correction device has been associated with a drawback of generating an error in the white balance correction in case where the distribution of color temperature of the object is not uniform or in cases where the object contains a large proportion of monochromatic area of a high saturation. In the following there is shown a representative example of such a drawback. In the following description, for the purpose of simplicity, the gain of the gain control circuits 3, 4 shown in FIG. 1 is assumed to be unity.
As an example of the above-mentioned drawback, there will be explained a case of taking white balance on an object 1 shown in FIG. 4, composed of white color by 50% and blue color by 50%.
It is assumed that the object 1 is illuminated with light of a high color temperature, for example of 9000.degree. K. In FIG. 6, V.sub.4r, V.sub.4b indicate the values of the control signals R.sub.cont, B.sub.cont for bringing the white point to the center of the vector chart shown in FIG. 5. When the object 1 is taken with the properly corrected white balance at R.sub.cont =V.sub.4r and B.sub.cont =V.sub.4b, the white color and the blue color are respectively positioned at W.sub.0, B.sub.0 in FIG. 5.
If the conventional white balance correction device 14 is activated in this state, the negative feedback loop thereof so functions as to reduce the (R-B) signal component of the object to zero, as explained before. Consequently, on the vector chart shown in FIG. 5, the white and blue points are moved upwards, parallel to the (R-B) axis (namely perpendicularly to a line R-B=0). Thus, when the negative feedback operation becomes stable, and if the limiting circuit 12 is assumed inactive, the white and blue points finally stay at positions W.sub.1, B.sub.1 satisfying a condition: line segment B.sub.0 B.sub.1 =line segment B.sub.1 A=line segment W.sub.O W.sub.1, wherein the line segment B.sub.0 A is parallel to the (R-B) axis, and the point A is on a line R-B=0 which passes through the original point W.sub.0 and is perpendicular to the (R-B) axis. The upward movements of the white and blue points in FIG. 5 correspond to increases in the control signals R.sub.cont and B.sub.cont, and the signals R.sub.cont, B.sub.cont required to move the white and blue points to W.sub.1, B.sub.1 are V.sub.7r, V.sub.7b in FIG. 6.
In practice, however, the control signals R.sub.cont, B.sub.cont are limited respectively to V.sub.3r, V.sub.3b by the function of the limiting circuit 12, so that, after the correction of white balance, the white and blue points do not move to W.sub.1, B.sub.1 on FIG. 5 but respectively remain at W.sub.2, B.sub.2. The aberrations of the white point W.sub.2 and the blue point B.sub.2 respectively from W.sub.0, B.sub.0 (lengths of line segments W.sub.0 W.sub.2 and B.sub.0 B.sub.2) in this state are respectively defined by the differences of the signals R.sub.cont, B.sub.cont from the ideal values, namely (V.sub.3r -V.sub.4r) and (V.sub.3b -V.sub.4b).
In this case, the control signals R.sub.cont, B.sub.cont which should read V.sub.7r, V.sub.7b as explained above are limited to V.sub.3r, V.sub.3b by the effective function of the limiting circuit 12, so that the aberrations of the white point W.sub.2 and the blue point B.sub.2 from W.sub.0, B.sub.0 do not become excessively large.
However the limiting circuit 12 does not function effectively for example in the following two cases, and the correction error of the white balance becomes not negligible in such situations.
(1) Let us consider a situation where the object 1 is illuminated with light of a low color temperature, for example of 2000.degree. K. In such state the values of the control signals R.sub.cont, B.sub.cont which bring the white point to the center of the vector chart shown in FIG. 5 are V.sub.2r, V.sub.2b as shown in FIG. 6. On the other hand, if said object 1 is taken under the function of the white balance correction device 14, the negative feedback loop thereof functions so as to reduce the (R-B) signal component of the object to zero, as explained before. Consequently, when said negative feedback operation is stabilized, the white and blue points eventually reach, as in the foregoing example, positions W.sub.1, B.sub.1 satisfying a condition: line segment W.sub.0 W.sub.1 =line segment B.sub.1 A=line segment B.sub.0 B.sub.1.
In this state, the signals R.sub.cont, B.sub.cont released from the white balance correction device 14 have values V.sub.5r, V.sub.5b shown in FIG. 6.
Thus, since the control signals R.sub.cont, B.sub.cont from the white balance correction device 14 are V.sub.5r, V.sub.5b instead of proper V.sub.2r, V.sub.2b, the white and blue points on FIG. 5 are aberrated from W.sub.0, B.sub.0 by line segments W.sub.0 W.sub.1 and B.sub.0 B.sub.1 corresponding to the differences (V.sub.5r -V.sub.2r) and (V.sub.5b -V.sub.2b) in said control signals. In this case, since there is no limit for preventing the control signals R.sub.cont, B.sub.cont from increasing to V.sub.5r, V.sub.5b, the white and blue points W.sub.1, B.sub.1 are aberrated from the properly corrected positions W.sub.0, B.sub.0 more significantly than in the foregoing example, so that the originally white area appears as pale orange while the originally blue area appears as pale blue.
(2) Also an object 2 consisting of a white area by 50% and a yellow area by 50% as shown in FIG. 7 leads to the following drawback.
It is assumed that said object 2 is illuminated with light of a high color temperature, for example of 9000.degree. K. In such state, the values of the control signals R.sub.cont, B.sub.cont bringing the white point to the center of a vector chart shown in FIG. 8 are V.sub.4r, V.sub.4b shown in FIG. 6. Thus, when the object 2 is taken with the proper white balance correction at R.sub.cont =V.sub.4r and B.sub.cont V.sub.4b, the white and yellow points appear at W.sub.0, Ye.sub.0 shown in FIG. 8.
If the white balance correction device 14 is activated in this state, the negative feedback loop thereof so functions as to reduce the (R-B) signal component of the object to zero, so that the white and yellow points eventually reach positions W.sub.3 and Ye.sub.1 satisfying a condition: line segment W.sub.0 W.sub.3 =line segment Ye.sub.1 B=line segment Ye.sub.0 Ye.sub.1, when said negative feedback operation is stabilized.
In this state, the control signals R.sub.cont, B.sub.cont released from the white balance correction device 14 have values V.sub.6r, V.sub.6b shown in FIG. 6.
Thus, since the control signal R.sub.cont, B.sub.cont from the white balance correction device 14 are V.sub.6r, V.sub.6b instead of proper V.sub.4r, V.sub.4b, the white and yellow points on FIG. 8 are aberrated from W.sub.0, Ye.sub.0 by line segments W.sub.0 W.sub.3 and Ye.sub.0 Ye.sub.1 corresponding to the differences (V.sub.6r -V.sub.4r) and (V.sub.6b -V.sub.4b) in said control signals. In this case, since there is not limit for preventing the control signals R.sub.cont, B.sub.cont from decreasing to V.sub.6r, V.sub.6b, the white and yellow points W.sub.1, Ye.sub.1 are aberrated from the properly corrected positions W.sub.0, Ye.sub.0 significantly as in the foregoing case (1), so that the originally white area appears bluish and the originally yellow area appears paler.
FIG. 9 shows the configuration of another prior art image pickup apparatus capable of further reducing the undesirable influence of a single object of high saturation on the white balance control. In FIG. 9, same components as those in FIG. 1 are represented by same numbers.
The image pickup apparatus shown in FIG. 9 is designed to only extract signals suitable for white balance control.
FIG. 10 is a color difference vector representation of the color video signal, for explaining the signals extracted in the apparatus of FIG. 9. If a color video signal obtained by taking a white object at a color temperature of 10000.degree. K. with appropriate white balance corresponds to a point P0, a color video signal obtained from the same object taken at a color temperature of 3000.degree. K. corresponds to a point P1.
On the other hand, if a color video signal obtained by taking the white object at a color temperature of 3000.degree. K. with appropriate white balance corresponds to the point P0, a color video signal obtained from the same object at a color temperature of 10000.degree. K. corresponds to a point P2.
Thus, the color of the color video signal varies along a thick line A in FIG. 10 when the white object changes in the color temperature.
When this color difference vector is represented in a two-dimensional coordinate system: EQU X=(R-Y)-(B-Y)=R-S EQU y=(R-Y)+(B-Y)=R+B-2Y,
y-coordinate is little affected by the color temperature, and x-coordinate alone varies by the color temperature.
Let us consider, then, to control the white balance in a color temperature range of 3000.degree. to 10000.degree. K. as explained above. The variation of a white or almost white object, in response to a change in the color temperature of 3000.degree. to 10000.degree. K., can be anticipated within a range (c.gtoreq.x.gtoreq.d) in the x-direction and a range (a.gtoreq.y.gtoreq.b) around the thick line A in the y-direction, or a hatched area SI in FIG. 10.
The image pickup apparatus shown in FIG. 9 is designed according to such concept, and the function of said apparatus will be explained in the following.
Light from an object, entering through a lens 21 and an iris (diaphragm) 22 is photoelectrically converted in an image pickup device 1, and a signal obtained by said photoelectric conversion is supplied to a luminance signal/chromaticity signal generating circuit 2, which generates a high frequency component YH and a low frequency component YL of Y signal, an R signal and a B signal. Among these signals, the R and B signals are respectively supplied to gain control circuits 3, 4.
The gain control circuits 3, 4 respectively amplify the R and B signals with gains determined by control signals R.sub.cont, B.sub.cont supplied from an automatic white balance correction circuit 38, thus releasing gain controlled signals R', B'.
The R' and B' signals are supplied, together with the YL signal, to a color difference signal generating circuit 5, which generates two color difference signals (R-Y) and (B-Y). Said color difference signals are supplied, together with the YH signal, to an encoder 6 for conversion into a standard television signal, which is released from a terminal 23.
The color difference signals (R-Y), (B-Y) are also supplied to said automatic white balance correction circuit 38, which will be explained in the following.
The color difference signals (R-Y), (B-Y) are respectively supplied to clamping circuits 27, 28 for matching the DC level thereof. Thereafter said signals are supplied to a subtraction circuit 29 and an addition circuit 30. The subtraction circuit 29 calculates the difference of the color difference signals (R-Y), (B-Y) from the clamping circuit 27, 28, thereby generating the above-mentioned signal x (=R-B). On the other hand, the addition circuit 30 calculates the sum of said signals thereby generating the above-mentioned signal y (=R+B-2Y).
Comparators 31, 32 compare the y signal with reference levels corresponding to a, b in FIG. 10. The comparator 31 releases a low (L) or high (H) level output signal for supply to an OR circuit 33, respectively if a .gtoreq.y or a&lt;y. The comparator 32 releases a low or high level output signal respectively if y.gtoreq.b or y&lt;b. Consequently the output of the OR circuit 33, controlling a gate circuit 34, assumes a low level state only when the y signal is in a range a.gtoreq.y.gtoreq.b, but otherwise assumes a high level state.
On the other hand, the x signal released from the subtraction circuit 29 is intercepted or transmitted by the gate circuit 34 respectively when the output signal of the OR gate 33 is the high or low level state. Thus the output signal from the gate circuit 34 is supplied to a control signal generating circuit 36, after clipping of portions c&lt;x and d&gt;x by a clipping circuit 35.
The control signal generating circuit 36 generates a correction signal z for controlling the gain control circuits 3, 4 in such a manner that the average of the input signal becomes equal to a reference potential, corresponding to a state with white balance.
The correction signal z is supplied to a tracking correction circuit 13, and is corrected therein so as to effect white balance control along the trajectory of the color video signal responding to the change of color temperature, thereby providing the control signals R.sub.cont and B.sub.cont, which control the gains of the gain control circuits 3, 4.
In the image pickup apparatus of the above-explained configuration, since the signal x from the subtractor 29 is extracted only when the signal y is positioned within a range b.ltoreq.y.ltoreq.a, so that the white balance control signal z is not affected by a colored object portion. Also thus extracted signal x is limited within a predetermined range by the clipping circuit, and is prevented from unnecessary gain control. Based on these facts, the white balance correction can be attained without the influence of the colored objects.
However, in the above-explained image pickup apparatus shown in FIG. 9, the colored and colorless objects are clearly distinguished by a predetermined boundary, and, whether the white balance can be attained depends on a slight difference in the saturation or hue. Also the hue of the output color video signal may vary by a slight change of the object.
For example, in case the object is composed of orange and blue colors, the color difference vectors for orange and blue respectively correspond to points Pa and Pb shown in FIG. 11. Since these points are both positioned within the above-mentioned hatched area SI, the white balance becomes stabilized in this state. However, if the orange color difference vector varies to a point Pa' outside said hatched area SI in FIG. 11, the signal from the original object portion is reflected in the white balance correction.
Consequently the white balance correction is conducted solely with the blue object portion, whereby the color difference vector representing the blue object portion is shifted from the point Pb to Pb', and the white balance becomes aberrated. This means that the white balance correcting function is significantly affected by a slight change in the hue, resulting from a variation in the image frame at the phototaking operation, a fluctuation among the cameras or a movement of the object.
The white balance correction is required for responding to the variation in the color temperature, but the range of video signal corresponding to a nearly white object also varies with such variation of the color temperature. Since the image pickup apparatus shown in FIG. 9 only employs the signals within the hatched area SI for white balance correction, the video signal corresponding to a same object may be employed or not for white balance correction, depending on the color temperature.
For this reason, the white balance correction may become different from the desired correction, depending on the color temperature. More specifically, depending on the color temperature, a colored object may have a color difference vector within the hatched area. For example, under illumination of a high color temperature, the light from the object tends to have a color difference vector with an enhanced blue component, whereby the color difference vector of a reddish object often enters the hatched area, and said reddish color is not reproduced but appears in faded state. Similarly, under a low color temperature, the bluish colors appear faded.